Swelling in horses’ lower legs is relatively common, however it can be alarming, and it is certainly worth horse owners knowing why it occurs and how to manage it. The medical term for this type of swelling is oedema, and it isn’t a swelling of blood or plasma, it is actually a build-up of something called interstitial fluid. To fully understand why oedema occurs in horses, we need to explore what interstitial fluid is, how it is formed and why a build-up can occur.

 

What is interstitial fluid and where does it come from?

 When blood is pumped around a horse’s arteries, it is at a high pressure. As the blood reaches small arterial blood vessels and even smaller capillaries which supply oxygen and nutrients to the animal’s organs and tissues, it is still at such a high pressure that some of the fluid is squeezed into the spaces between the cells. Red blood cells, which give blood its characteristic colour, and other proteins are too large to pass through the walls of small blood vessels, so interstitial (which in Latin means “between place”) fluid is the salty, almost clear filtered version of blood which does makes it through to sit around the cells. In a normal, healthy horse interstitial fluid comprises around 15% of their overall fluids.

 Interstitial fluid is slowly cleared by the lymphatic system, part of the vascular or circulatory system. Once it enters the lymphatic system, interstitial fluid is referred to as ‘lymph’, similar to the Latin word for water. Lymphatic vessels are separate from but run close to blood vessels and eventually, thanks to either small contractions of the vessels themselves (a little like peristalsis occurring the intestinal walls to move food through the digestive system) or external compression for muscular contractions and movement, lymph is returned back into the blood via the veins.  

 

Why does oedema occur?

 Swelling or filling in the lower legs, also known as oedema, occurs when either the production of lymph is greater than normal, or the clearance of lymph by the lymphatic system is slower than normal – or, both occur at the same time.

 Both the slowing of the lymphatic system and excessive production of lymph can be triggered by a wide range of factors including chronic lung disease, kidney disease, heart failure, liver disease, thyroid issues, damaged blood vessels, nerves or lymphatic vessels, medication, parasites, injuries, burns and bites. This enormous range of possible causes sounds scary, but in the otherwise healthy horse (and if all four legs swell at the same time), the most common cause is likely to be inactivity.

 

How can you prevent or manage oedema?

 Filling of the legs is seen most frequently in horse who have spent a lot of time stabled, for example horses on box rest or horses who are stabled overnight. In the wild, equines will spend most of the day walking as they graze or seek out new grazing and water. This constant movement means the lymphatic system has the assistance of compression from the horse’s muscles to keep removing lymph so when a horse is inactive for long periods, fluid may build up. Horses also have no muscle below the knee to provide compression and release on the lymphatic system, so this area of the body is quite susceptible to oedema.

 If inactivity is the cause, the filling should dissipate once the horse is able to move around freely again. If this isn’t possible, perhaps due to strict box rest, or if the swelling seems to be causing pain then hosing and bandaging can alleviate the swelling. Massaging can assist the lymphatic system in moving lymph back up the leg if there is a build-up and hosing and bandaging can be helpful for horses who repeatedly present with filled legs when stabled.

 Sudden filling in just one leg could be a symptom of cellulitis, a bacterial infection of the skin and underlying tissues, or an insect bite. Cellulitis can be serious if left untreated, so it is worth assessing, monitoring and involving your vet if needed. Allergic reactions causing oedema can affect all 4 legs, so if an otherwise healthy horse presents with filled legs when its movement has not been restricted, it is worth looking for possible triggers of an allergic reaction, such as nettles.

 The ComfortStall flooring system from Haygain can help prevent filled legs, thanks to its level, supportive surface that ‘springs’ back with every step the horse takes. There are many additional benefits to ComfortStall flooring that can help to boost the health and wellbeing of your horse. Take a look at them here.

 With thanks to Dr David Marlin and Haygain for their knowledge and expertise.